In antenna design, the space available is an important factor. It is relatively easy to make an antenna of good quality if there are no size limitations. In small-sized radio devices, such as mobile phones, an antenna which protrudes outside the covers of the device is tried to be avoided, for convenience. This means that as the devices become smaller and smaller, the space available for the antenna becomes smaller, too, making antenna design even more challenging.
Internal antennas in mobile terminals usually have planar structures: The antenna comprises a radiating plane and a ground plane parallel thereto. The electrical characteristics of a planar antenna, such as bandwidth and antenna gain, depend on the distance between said planes, among other things. As mobile terminals become smaller in the direction of thickness, too, this distance inevitably becomes shorter, whereby the electrical characteristics become worse. Particularly this problem concerns foldable mobile phone models, as their folding parts are relatively flat and thin. Therefore, antennas in foldable models are in practice protruding antennas.
Available space can be used more efficiently in a radio device by fabricating the radiating element of the antenna within the cover of the device or as part of the cover, which is known as such. FIG. 1 shows an example of a radiating antenna element known from application FI20012219 which element is intended to be part of a cover of the radio device. Strictly speaking the planar bottom 110 of the antenna element 100 and its curved rim 120 are included in the cover of the device. The rim is found on three sides of the bottom, corresponding to an end of the radio device and the side surfaces at that end. When mounted, the element 100 is a radiating element in a planar inverted F antenna (PIFA), where an antenna feed conductor and short-circuit conductor are connected to the element. Antenna feed point 101 and short-circuit point 102 are marked as broken-line circles on the bottom 110. When the antenna element 100 is pressed into its place in the radio device the feed and short-circuit conductors make galvanic contact with points 101 and 102. Starting from the edge of the element 100 there is a slot 105 which makes a rectangular turn such that the element, viewed from the short-circuit point 102, is divided into two branches of different lengths. The antenna is thus a dual-band antenna. On both sides of the portion of the slot 105 which starts from the edge of the element there is a capacitance plate perpendicular to the bottom. A first capacitance plate 131 is located at the electrically outermost end of the longer branch of the element, and a second capacitance plate 132 at the electrically outermost end of the shorter branch. Both the mutual capacitance of the capacitance plates and their capacitances with the ground plane (not shown) increase the electrical lengths of the radiating branches. This reduces the size of an antenna operating in particular frequency bands. Furthermore, the antenna element 100 includes, protruding from the surface of the bottom 110, a support leg 141 and a ridge 142 which resembles a wide U and adds to the mechanical strength of the antenna element. For attachment of the antenna element it further comprises locking parts 151 and 152. All parts may be included in a single extrusion piece.
A disadvantage of the element shown in FIG. 1 is that its parts have to have certain electrical sizes, which limits the design of the element. Moreover, the characteristics of the antenna using the element may be inadequate in flat and thin radio devices.